The Origin and Significance of Cryptobioturbation in Alluvial Plain and Coastal Plain Channel Deposits, Late Cretaceous Neslen, Price River, and Williams Fork Formations, Uinta and Piceance Basins, Eastern Utah and Western Colorado

Edmund R. Gustason
El Paso Exploration & Production, Denver, CO

Alluvial plain and coastal plain channel deposits of the Late Cretaceous Neslen, Price River, and Williams Fork formations are prolific tight gas sandstone producers in the Uinta and Piceance basins of eastern Utah and western Colorado. Channel deposits contain a wide variety of facies, including mud-clast conglomerate, high- and low-angle cross stratified sandstone, planar laminated sandstone, ripple cross laminated sandstone, and massive or featureless sandstone, and heterolithic sandstone, siltstone and mudstone. This paper focuses on the origin and significance of the massive or featureless sandstone facies.

Previous work has interpreted massive or featureless sandstone facies as mass flow deposits due to bank collapse or rapid sedimentation. This facies is reinterpreted as pervasive cryptobioturbation. Discrete trace fossils are very small (< 1mm in diameter), cylindrical with a clay core and clean rim, and similar to Helminthopsis and Anconichnus. Pervasive cryptobioturbation has created a homogenous fabric, obscuring original lamina-scale and bed-scale heterogeneities. Pervasive cryptobioturbation typically crosses boundaries between and/or grades downward into sandstone facies with preserved primary physical structures and is sharply overlain (truncated) by sandstone facies with primary physical structures. This mostly monospecific assemblage occurs with rare Arenicolites, Planolites, and small forms of Teichichnus and Ophiomorpha, suggesting a brackish water or “stressed” depositional environment.

Pervasive cryptobioturbated sandstone has up to 3% higher porosity and better permeability than other sandstone facies. This is, in part, due to preservation of some primary porosity by clay coatings that protected framework grains from extensive quartz cementation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas